The youngest actor to ever play The Doctor, 26 year old Matt Smith, will be taking the lead as the legendary Time Lord from current actor David Tennant, next year.
Doctor Who is about the adventures of a mysterious time-traveller known as the Doctor. The Doctor travels through space and time in a craft known as the TARDIS, an acronym for "Time and Relative Dimension in Space". The Doctor is usually accompanied by one or more companions, who are often attractive females. The tone of the programme varies from serious to comic, from gothic horror to pantomime camp. The original Doctor Who series is fondly remembered among the general public both for frightening monsters (such as the Daleks and Cybermen) and pioneering use of both electronic music and low-budget special effects. http://tardis.wikia.com/wiki/Doctor_Who
David Tennant is great and Christopher Ecclestone is a really underappreciated actor as well. I'd never actually heard of Matt Smith before but going by what I have seen of him in interviews over the past couple of days I think he'll do alright. He seems to naturally have a pretty eccentric demeanour. He does look almost 10 years younger than he actually is though, haha.